1. Field of Invention
The present invention is directed to ball seats for use in oil and gas wells and, in particular, to ball seats having one or more fluid activated ball support.
2. Description of Art
Ball seats are generally known in the art. For example, typical ball seats have a bore or passageway that is restricted by a seat. The ball or drop plug is disposed on the seat, preventing fluid from flowing through the bore of the ball seat and, thus, isolating the tubing or conduit section in which the ball seat is disposed. As the fluid pressure above the ball or drop plug builds up, the conduit can be pressurized for tubing testing or actuating a tool connected to the ball seat such as setting a packer. Ball seats are also used in cased hole completions, liner hangers, flow diverters, frac systems, and flow control equipment and systems.
Although the terms “ball seat” and “ball” are used herein, it is to be understood that a drop plug or other shaped plugging device or element may be used with the “ball seats” disclosed and discussed herein. For simplicity it is to be understood that the term “ball” includes and encompasses all shapes and sizes of plugs, balls, or drop plugs unless the specific shape or design of the “ball” is expressly discussed.
As mentioned above, all seats allow a ball to land and make a partial or complete seal between the seat and the ball during pressurization. The contact area between the ball and the inner diameter of the seat provides the seal surface. Generally, the total contact area or bearing surface between the ball and the seat is determined by the outer diameter of the ball and the inner diameter of seat. The outer diameter of the contact area is determined by the largest diameter ball that can be transported down the conduit. The inner diameter of the seat is determined by the allowable contact stress the ball can exert against the contact area and/or the required inner diameter to allow preceding passage of plug elements or tools, and/or subsequent passage of tools after the plug element is removed, through the inner diameter of the seat.
The seat is usually made out of a metal that can withstand high contact forces due to its high yield strength. The ball, however, is typically formed out of a plastic material that has limited compressive strength. Further, the contact area between the ball and seat is typically minimized to maximize the seat inner diameter for the preceding passage of balls, plug elements, or other downhole tools. Therefore, as the ball size becomes greater, the contact stresses typically become higher due to the increasing ratio of the cross-section of the ball exposed to pressure compared to the cross-section of the ball in contact with the seat. This higher contact pressure has a propensity to cause the plastic balls to fail due to greater contact stresses.
The amount of contact pressure a particular ball seat can safely endure is a direct function of the ball outer diameter, seat inner diameter, applied tubing pressure, and ball strength. Because of limited ball strength as discussed above, the seat inner diameter is typically reduced to increase the contact area (to decrease contact stress). The reduced seat inner diameter forces the ball previously disposed through the seat inner diameter to have a smaller outer diameter to pass through this seat inner diameter. This reduction in outer diameter of previous balls continues throughout the length of conduit until ball seats can no longer be utilized. Therefore, a string of conduit is limited as to the number of balls (and, thus ball seats) that can be used which reduces the number of actuations that can be performed through a given string of conduit.